1. Field of the Invention
The present invention relates to a process for manufacturing a triaxial piezoresistive accelerometer and the relative pressure-monitoring device, in particular a device for monitoring the pressure of the tires of a motor vehicle, to which the ensuing description will make explicit reference, without any loss of generality.
2. Description of the Related Art
As known, in the automotive field there is an increasing use of devices for monitoring the pressure of tires (generally known as tire-pressure monitoring systems—TPMSs), which are designed to supply a timely communication, to the electronic control unit of the vehicle, of any fault or deviation with respect to the correct values of operation. These monitoring devices generally comprise a pressure sensor installed on the inner surface of the tire and designed to monitor its state of inflation; an appropriate electronic circuit, which reads the data provided by the pressure sensor and communicates with the electronic control unit (generally using radio-frequencies); and a wake-up system, which supplies a start-of-measurement signal to the pressure sensor and a data-collection signal to the electronic circuit connected thereto. In particular, the wake-up system makes it possible to limit the monitoring operation to the time intervals when the vehicle is moving (it is estimated that the average time of effective use of a vehicle is around 5% of the total life of the vehicle), and thus to reduce the energy consumption by the vehicle battery. Known wake-up systems are either of a mechanical type, generally formed by a mass coupled to a spring, or, as in the case of more recent systems, of an electronic type. Wake-up systems of an electronic type comprise an accelerometer arranged so as to detect the centrifugal acceleration of the tire as it turns. An acceleration of intensity higher than a preset threshold indicates a movement condition of the vehicle.
The various components of the device for monitoring pressure are currently made using different technologies and subsequently assembled on an electronic board. The device is then coated with resin and individually packaged. Consequently, the pressure-monitoring device is currently cumbersome (around 10 mm in size) and somewhat complex to produce.
Recently, the use has been proposed, within the pressure-monitoring device, of semiconductor piezoresistive accelerometers made using microfabrication techniques.
As is known, piezoresistive sensors base their operation on piezoresistivity, i.e., the capacity of certain materials to modify their resistivity as the mechanical stresses acting on them vary. In detail, the resistivity decreases when compressive stresses are applied, whereas it increases when tensile stresses are applied.
Semiconductor piezoresistive accelerometers generally comprise a membrane (or diaphragm) suspended over a cavity, and an inertial mass fixed to the membrane, and mobile with one or more degrees of freedom after detecting an acceleration. Piezoresistive elements (generally formed by implanted or diffused regions) are made in the surface region of the membrane and are connected to one another in a Wheatstone-bridge configuration. A deformation of the membrane, caused by the displacement of the inertial mass induced by the acceleration, causes an unbalancing of the Wheatstone bridge, which can be detected by a purposely provided electronic circuit, which derives, from said unbalancing, the desired measurement of acceleration.
A triaxial piezoresistive accelerometer of a known type is, for example, manufactured by Fujikura Ltd. and described in detail in “www.sensorsmag.com/articles/0299/0299—38/main.shtml”.
This accelerometer is illustrated in FIG. 1, where it is designated as a whole by reference number 1. The accelerometer 1 comprises a first and a second silicon layer 2, 3, between which glass layer 4 is arranged. In particular, the layers are bonded to one another via anodic bonding, and the entire structure is enclosed in a ceramic package (not illustrated in FIG. 1).
In extreme synthesis, the manufacturing process of the accelerometer 1 envisages the diffusion of boron regions in the surface region of the first silicon layer 2 so as to form piezoresistive elements 6 that are connected in a Wheatstone-bridge configuration (not illustrated in FIG. 1). Then, the rear face of the first silicon layer 1 is anisotropically etched so as to form a thin silicon membrane 8. After the etch, a central portion 9 of the first silicon layer 2 remains underneath the membrane 8. Next, the glass layer 4 is bonded to the rear surface of the first silicon layer 2 via anodic bonding and the layer of glass 4 is cut on the rear side (opposite to the bonding side), so as to form an inertial mass 10 at the center of the structure of the accelerometer 1, underneath the membrane 8. In particular, the inertial mass 10 is etched only at the central portion 9. Finally, the second silicon layer 3 is bonded via anodic bonding underneath the layer of glass 4, which has the function of base and of mechanical protection for the accelerometer 1. At the inertial mass 10, the second silicon layer 3 has a cavity 12, appropriately made before bonding, so as to ensure freedom of movement to the inertial mass 10. The distance between the inertial mass 10 and the second silicon layer 3 is such as to limit the movement of the inertial mass 10 in a transverse direction, to prevent the membrane 8 from getting damaged in case of excessive accelerations.
An acceleration imparted upon the accelerometer 1 causes a displacement of the inertial mass 10, and a consequent deformation of the membrane 8. Due to this deformation, the piezoresistive elements 6 vary their resistivity, so unbalancing the Wheatstone bridge.
The accelerometer 1 described, even though it is certainly more compact than wake-up systems of a mechanical type, has in any case large dimensions on account of the need to carry out a bonding of three different layers (two layers of silicon and one layer of glass) and on account of the presence of a ceramic package, and entails a manufacturing process that is somewhat complex and costly. Furthermore, the accelerometer 1 cannot be readily integrated with the electronic read circuit. These disadvantages are particularly evident as regards the considered automotive applications, wherein low cost and simplicity of production are a constraint in the choice of the components to be used.